Importance: Prior infection and vaccination both contribute to population-level SARS-CoV-2 immunity. Population-level immunity will influence future transmission and disease burden.
Objective: For each US county and state, we estimated the fraction of the population with prior immunological exposure to SARS-CoV-2 (ever infected with SARS-CoV-2 and/or received one or more doses of a COVID-19 vaccine) as well as the fraction with effective protection against infection and severe disease from prevalent SARS-CoV-2 variants, from January 1st, 2020, to October 31st, 2021.
Design, settings, participants: We used daily SARS-CoV-2 infection estimates for each US state and county, derived based on reported data on COVID-19 cases and deaths. We collated county-level vaccination coverage data and estimated the fraction of individuals both vaccinated and previously infected using the Census Bureau Household Pulse Survey. We used published evidence on natural and vaccine-induced immunity, and how protection wanes over time. We used a Bayesian model to synthesize evidence and estimate population immunity outcomes.
Main Outcomes and Measures: Primary outcomes were the fraction of the population with (i) a history of exposure to SARS-CoV-2 infection or COVID-19 vaccination or both, (ii) effective protection against infection, and (iii) effective protection against severe disease. We estimated outcomes for each US state and county from January 1st, 2020, to October 31st, 2021.
Results: The estimated percentage of the US population with a history of SARS-CoV-2 infection or vaccination, as of October 31, 2021, was 86.2% (95%CrI: 82.2%-93.0%), compared to 24.9% (95%CrI: 18.5%-34.1%) on January 1, 2021. State-level estimates for October 31, 2021, ranged between 72.2% (95%CrI: 62.5%-83.3%, West Virginia) and 92.3% (95%CrI: 88.6%-96.1%, Florida). Accounting for waning, the effective protection against infection with prevalent strains as of October 31 was 49.9% (95%CrI: 45.4%-56.6%) nationally and ranged between 37.2% (95%CrI: 33.4%-44.7%, Vermont) and 59.5% (95%CrI: 56.4%-66.0%, Florida). Effective protection against severe disease was 77.4% (95%CrI: 73.7%-83.4%) nationally and ranged between 62.9% (95%CrI: 55.2%-73.3%, West Virginia) and 83.8% (95%CrI: 80.7%-88.0%, Florida).
Conclusions and Relevance: The fraction of the population with effective protection against SARS-CoV-2 infection and severe COVID-19 varies across the United States, but a substantial proportion of the population remains susceptible.